Using graphite nano-platelets to improve the integrity of oil and gas wells

ABSTRACT

Embodiments relate to use of graphite nanoplatelets (GnP) to enhance the mechanical and durability characteristics of cement that may be used as cement sheaths in wellbores of oil and gas wells. Generally, undesired permeability of cement is caused by diffusion of trapped oil and/or natural gas through the cementitious matrix of the cement, leading to material degradation of the cement. Methods disclosed involve using modified GnPs (having physically modified surfaces or chemically modified surfaces energies) to generate a cementitious nanocomposite with uniformly dispersed GnPs, which can effectively arrest the undesired diffusion mechanism. Modified GnPs can also increase the strength of interfacial adhesion (e.g., interfacial bonds and interfacial energies) between the GnP and the cement matrix (e.g., hydrations of the cement). Physical modification of GnP can involve non-covalent treatment techniques. Chemical modification of GnP can involve covalent treatment techniques.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of U.S.Provisional Application no. 62/827,464, filed on Apr. 1, 2019, theentire contents of which is incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments relate to the use of modified graphite nanoplatelets(graphite nanoplatelets having physically modified surfaces orchemically modified surfaces energies) to enhance the mechanical anddurability characteristics of cement that may be used as cement sheathsin wellbores of oil and gas wells.

BACKGROUND OF THE INVENTION

Conventional systems and methods for improving mechanicalcharacteristics of cement sheaths and/or wellbores can be appreciatedfrom U.S. Pat. Nos. 6,457,524, 7,156,173, U.S. Pat. Publ. No.2008/0134942, U.S. Pat. Publ. No. 2009/0229494, U.S. Pat. Publ. No.2011/0107942, U.S. Pat. Publ. No. 2011/0067864, U.S. Pat. Publ. No.2017/0166722, New Cement Formulations Utilizing Graphene Nano Plateletsto Improve Cement Properties and Long-Term Reliability in Oil Wells byAlkhami et al. SPE-192342-MS, Society of Petroleum Engineers, 2018,avail, and The Use of Low-Cost Graphite Nanomaterials to Enhance ZonalIsolation in Oil and Gas Wells by Payvandi et al., SPE-187105-MS.Society of Petroleum Engineers, 2017. Conventional systems require useof expensive forms of nano-particle and nano-particle treatmenttechniques. In addition, conventional systems do not effectively provideadequate dispersion of nano-particles within the cementitious matrix.Moreover, conventional systems fail to provide adequate strength of theinterfacial bond and energy between nano-additives and the matrix. Theseand other disadvantages may limit the use of conventional systems.

BRIEF SUMMARY OF THE INVENTION

Embodiments relate to the use of nanoparticles to enhance the mechanicaland durability characteristics (e.g., integrity, ductility, toughness,compressive strength, tensile strength, flexural strength, shear bondstrength, fracture properties, nanoscale properties, microstructure,permeability, viscosity, Rheological properties, thickening time, freefluid movement, etc.) of cement that may be used as cement sheaths inwellbores of oil and gas wells. Nanoparticles can be configured to havelarge surface areas and high aspect ratios, which can increase the vander Walls interaction between particles. This aspect of thenanoparticles can be exploited to influence mechanical and othermaterial properties of cement if the nanoparticles are used as additivesin the cement, thereby making nanoparticles attractive candidates asadditives used in cement. Use of nanoparticles for this purpose is knownto those skilled in the art.

Embodiments disclosed herein relate to the use of modified graphitenanoplatelets (graphite nanoplatelets (GnP) having physically modifiedsurfaces and/or chemically modified surfaces energies) to enhance themechanical and durability characteristics of cement or cementitiousnanocomposite used to make the cement. GnPs have large surface areas andhigh aspect ratios, and thus modifying their surface chemistry can be ameans to control dispersion uniformity and interfacial adhesion of GnPswhen used as additives in the cement. It should be noted that otherdispersion techniques (e.g., mechanical dispersion via ultrasonicationand high shear stirring) can be used to augment any of the dispersionmethods disclosed herein. The modified GnPs can be used to: 1) influencethe uniformity with which the GnPs are dispersed within the cementand/or cementitious nanocomposite; and 2) influence the interfacialadhesion between the modified GnP and the cement matrix. Theconcentration of GnPs within the cement and/or cementitiousnanocomposite can be adjusted to further enhance the mechanical anddurability characteristics.

Uniform distribution of GnPs throughout the cement can inhibit orprevent diffusion of material (e.g., trapped oil and/or natural gas)through the pore network and the micro-cracks of the cement. Diffusionof the material through the cement can otherwise lead to materialdegradation of the cement. Uniform dispersion of GnPs inhibits orprevents this diffusion mechanism, because when GnPs are not dispersedcompletely they interact with each other through the electrostaticforces, leading to the formation of a flocculated structure. In a slurrycement formation, the weight of the cementitious material is typicallytransmitted to the bottom of slurry by the gel lattice of the cement.Consequently, a structural deformation happens within the cement,whereby water is squeezed out of the lower parts of the slurry andmigrates upward the less-stressed upper layers. The capacity of thehigher layers to accommodate the additional water is limited, resultingin a layer of water accumulating at the top of the cement slurry. Thisseparation and movement of water from the bottom of the slurry upwardthe higher layers can cause channels within the cement, which willpromote the probability of gas migration and impair zonal isolation. Inother words, the free water coalesces to form a continuous channel onthe upper side of the cement sheath and develops a path by which the oiland gas can migrate. Modifying the surface energies of the GnPs canimprove the uniformity with which the GnPs are dispersed, mitigatingthis diffusion effect.

Modified GnPs enhance interfacial adhesion by generating higher bondstrength between the modified GnPs and the hydrations (C—S—H, calciumhydroxide, etc.) of cement. Improved interfacial adhesion can inhibit orprevent debonding of the cement from the casing and/or the surroundingrock formation of the wellbore.

Uniform dispersion and increased interfacial adhesion, along withadjustment of GnP concentrations, can be used to generate a cementitiousnanocomposite that may be used as a component to cement such that thecement exhibits the enhanced mechanical and durability properties thatare desirous for use as cement sheaths in wellbores.

It should be noted that conventional methods rely on the use of carbonnanotubes (CNTs) or carbon nanofibers (CNFs), and also focus on uniformdispersion of CNTs and CNFs in cementitious nanocomposites. CNTs andCNFs, however, are very expensive ($500 per pound), which render themimpractical for oilfield applications. In addition, cements made withCNT- or CNF-cementitious nanocomposites require implementation ofburdensome curing procedures.

It should be further noted that conventional dispersion techniques relyon mechanical approaches that exploit the intrinsic hydrophobicity ofgraphitic surfaces. As will be explained, embodiments of the dispersionmethods disclosed herein use physical modified surfaces and/or chemicalsurface modification of GnPs to introduce hydrophilic groups on the freesurfaces of graphite sheets.

Further features, aspects, objects, advantages, and possibleapplications of the present invention will become apparent from a studyof the exemplary embodiments and examples described below, incombination with the Figures, and the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

The above and other objects, aspects, features, advantages and possibleapplications of the present invention will be more apparent from thefollowing more particular description thereof, presented in conjunctionwith the following drawings, in which:

FIG. 1 is a schematic presentation of an embodiment of GnP.

FIG. 2 is a schematic presentation of GnP modified via physical surfacealteration.

FIG. 3 is a schematic presentation of GnP modified via chemical surfaceenergy alteration.

FIG. 4 shows functionalized GnP at different stages of acetone washing.

FIG. 5 shows a prepared wet filter cake of GnPs after polymer wrapping(physical surface alteration).

FIG. 6 is a graph showing compressive behavior of nanocomposite cementpastes fabricated with chemically modified GnPs.

FIG. 7 is a graph showing compressive behavior of nanocomposite cementpastes fabricated with physically modified GnPs.

FIGS. 8A-8C are graphs showing mathematical models of shear stress-shearrate curves at room temperature, T=120° F., and T=190° F., respectively.

FIGS. 9A-9B are graphs showing shear stress-shear rate curves of aprepared cement slurry at different temperatures and their correspondingviscosity, respectively.

FIGS. 10A-10C are graphs showing HPHT thickening time results for 0.1,0.2, and 0.4 Vol. % acid-functionalized GnPs, respectively.

FIG. 11 is a graph comparing consistency of different cement sampleshaving different concentrations of acid-functionalized GnPs.

FIGS. 12A-12B illustrate an exemplary experimental set-up for a modifiedpush-out tests, and dimension of steel casing used in the set-up,respectively.

FIG. 13 is a graph comparing push-out test results for plain cement andcement including different concentrations of acid-functionalized GnP,0.1, 0.2, and 0.4 vol. %.

FIG. 14 is a graph comparing flexural test (three-point bending test)results for plain cement and cement including different concentrationsof acid-functionalized GnP, 0.1, 0.2, and 0.4 vol. %.

FIGS. 15A-15D illustrate the scanning electron microscopy (SEM) imagesof an exemplary for the microstructural effects of chemicallysurface-modified GnPs on pore refinement. Images are taken from thefracture surface of cement composed of 0.2 vol. % GnPs after conductingflexural test.

FIG. 16 illustrates an exemplary experimental set-up for injection testto measure the depth of slurry penetration into a channel with thethickness of 120 μm and width of 1 in.

FIGS. 17A-17B are graphs comparing the capability of cement slurries topenetrate narrow spaces for plain cement and cement including 0.2 vol. %acid-functionalized GnPs.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of an embodiment presently contemplated forcarrying out the present invention. This description is not to be takenin a limiting sense, but is made merely for the purpose of describingthe general principles and features of the present invention. The scopeof the present invention should be determined with reference to theclaims.

Embodiments can include a cementitious nanocomposite that may be used asa component of cement. It is contemplated for the cement to be used ascement sheaths for supporting casings that are used in wellbores of oiland gas wells; however, the cement can be used for other applications.The cementitious nanocomposite can include graphite nanoplatelets (GnP)dispersed within the cementitious nanocomposite. For example, thecementitious nanocomposite can include a binder (e.g., lime, calciumsilicate, etc.), aggregate (e.g., sand, gravel, etc.) and an additive(e.g., GnP). While other materials, compositions, and additives can beused to generate the cementitious nanocomposite, it is contemplated forGnP to be used as at least one of the additives.

The distribution of the GnPs throughout the cementitious nanocompositecan be uniform so as to inhibit or prevent the diffusion ofenvironmental materials (e.g., trapped oil and/or natural gas) throughthe pore network and the micro-cracks of cement formed by thecementitious nanocomposite. The distribution of the GnPs can beinfluenced by surface energy modification of the GnPs. Surface energymodification of the GnPs can also affect the strength of interfacialadhesion (e.g., interfacial bonds and interfacial energies) between theGnP and the cement matrix (e.g., hydrations of the cement). The abilityto inhibit or prevent diffusion of environmental materials and theability to increase the interfacial adhesion can improve the mechanicaland durability properties of the cement, and in particular theproperties typically desired for cement sheaths.

Additional embodiments can include methods of making and using cement(or cementitious nanocomposite for the cement), the method involvingdispersing GnPs within a cementitious nanocomposite. The method caninvolve modifying the surface energy of the GnPs via chemical and/or thesurface of the GnP via physical modification to generate modified GnPs.Use of modified GnPs can influence the dispersion of GnPs within thecementitious nanocomposite, and more precisely, within the aqueousmedia. For instance, a cementitious nanocomposite with modified GnPs canhave a more uniform GnP distribution than a cementitious nanocompositehaving non-modified GnPs or having some other additive. Embodiments ofthe chemical surface energy alteration technique can involve oxygenfunctionalizing the surface of the GnPs, which may be done via acidfunctionalization. Embodiments of the physical surface alterationtechnique can involve polymer wrapping via different types of polymer,for example, a) poly acrylic acid (PAA); or b) a combination ofpolyvinyl pyrrolidone (PVP) and sodium dodecyl sulfate (SDS).

As noted herein, embodiments can relate to a cementitious nanocomposite,or methods of making and using the same, to be used as a component forcement. The cementitious nanocomposite can have modified GnPs added tothe cementitious nanocomposite. The cementitious nanocomposite may ormay not be mixed with other cementitious material or additives to formthe cement. As noted herein, it is contemplated for the cement to beused as a component for a cement sheath for a wellbore of an oil or gaswell. For instance, a wellbore can include a casing (e.g., a hollow pipeplaced inside the wellbore) that is cemented in place by introducing acement sheath around casing. The permeability of the cement can allowfor the diffusion of trapped oil and/or natural gas through the porenetwork and the micro-cracks of cementitious media of the cement,leading to environmental, safety, and structural concerns. For instance,the diffusion of trapped oil and/or natural gas can degrade themechanical properties of the cement. Embodiments of the cementitiousnanocomposite, and methods of making and using the same, can includeincorporating modified GnPs to inhibit or prevent this diffusionmechanism.

Referring to FIG. 1 , embodiments of the GnPs can include athree-dimensional (3D) structure composed of a few to several parallelgraphene sheets. The overall thickness of the GnP can be within therange of 1 nanometer. The theoretical surface area of the GnP can becalculated to be as high as 2630-2965 m²/g. In some embodiments, carbonatoms within each basal plane of the GnP possess sp² hybridized orbitalsthat are covalently bonded to three other adjacent atoms located in thesame plane. The fourth valence electron with a delocalized π orbital ispaired with another delocalized electron located on the neighboringgraphene layer. These π-π interactions weakly bond adjacent graphenesheets by van der Waals forces.

Embodiments of the GnP can be modified to generate modified GnP. Thiscan include modifying the surface energy of the GnP. This can beachieved by functionalizing the GnP surface to causing active sites ofthe GnP to adjust in hydrophilicity, hydrophobicity, surface charge,surface energy, etc. It is contemplated to modify the surface energy soas to improve the chemical compatibility of the GnP within their matrixby enhancing their wetting or adhesion characteristics and reducingtheir tendency to agglomerate (i.e., providing more uniform dispersion)while incorporated into the cementitious nanocomposite. Thismodification can also improve the bonding between GnP and the cementmatrix. As a none-limiting example, some embodiments can includefunctionalizing the GnP surface to introduce hydrophilic groups on thefree surfaces of the GnP sheet. Generating modified GnP can involvephysical surface alteration techniques and/or chemical surface energyalteration techniques.

Referring to FIG. 2 , physical surface alteration techniques can involvenon-covalent treatment of GnP. This can include non-covalent polymerwrapping techniques. In one embodiment, the non-covalent polymerwrapping involves physical adsorption of surfactants or charged polymersonto the surface of GnP.

Some embodiments of physical surface alteration can involve adsorptionof poly acrylic acid (PAA). For instance, GnPs can be added to deionizedwater to form a mixture, wherein PAA can be added to the mixture. Themixture may be sonicated and stirred. The GnPs of the mixture can befiltered to be separated out. The GnPs can be washed by deionized waterto remove excess or residual PAA.

Some embodiments of physical surface alteration can involve adsorptionof polyvinyl pyrrolidone (PVP) and sodium dodecyl sulfate (SDS). Forinstance, GnPs can be dispersed in deionized water with the aid of anSDS solution to form a mixture, wherein PVP can be mixed into themixture. The mixture can be sonicated and then incubated. The GnPs ofthe mixture can filtered to be separated out. The GnPs can be washedwith deionized water to remove excess or residual SDS.

After physical surface alteration, the modified GnP can be used as acomponent to form a cementitious nanocomposite. The adsorption ofsurfactants that occurs during physical surface alteration can enhancethe dispersion of modified GnP in the cementitious nanocomposite whenthe modified GnP is used to form that cementitious nanocomposite.Enhancing the dispersion of the GnP is defined herein as allowing orcausing the GnP to be more uniformly dispersed throughout thecementitious nanocomposite (and/or the cement when the cementitiousnanocomposite is used to form the cement). The dispersion of the GnP isenhanced due to the introduction of a small hydrophilic head group, abenzene ring, and/or a long alkyl chain (e.g., PVP and SDS). Thisimproves the chemical compatibility of the GnP with their matrix byenhancing their wetting or adhesion characteristics and reducing theirtendency to agglomerate while in the cementitious nanocomposite. Itshould be noted, however, the non-covalent polymer wrapping of the GnPdoes not affect the inherent π-bonds, and thus there is a positiveimpact that enhances bonding strength inside the cement matrix.

Referring to FIG. 3 , chemical surface energy alteration techniques caninvolve covalent treatment of GnP. This can include the formation ofcarboxyl and hydroxyl groups onto the surface of the GnP. In someembodiments, oxygen functionalization can be achieved via acidfunctionalization. In some embodiments, the oxygen functionalization canintroduce hydrophilic groups on the free surfaces of graphite sheets,thereby improving the chemical compatibility of the GnPs with theirmatrix by enhancing their wetting or adhesion characteristics andreducing their tendency to agglomerate.

Embodiments of chemical surface energy alteration can involve generatinga solution of nitric and sulfuric acids. GnP can be added to thesolution, followed by stirring. The GnPs can be washed by deionizedwater and HCl. The GnP can then be held still, wherein a top part of thediluted mixture is removed. This washing process may be repeated if thesulfate ions are still detectable in the diluted mixture. The techniquecan further include washing the GnPs by acetone to remove carboxylatedcarbonaceous fragments (CCFs), which are organic molecules havingcondensed aromatic graphitic rings with several functional groups. CFFsare formed during GnP functionalization. The GnPs can be iterativelywashed by centrifugation until a clean and colorless diluted mixture isobtained, indicating that the GnPs are free from CFFs.

After chemical surface energy alteration, the modified GnP can be usedas a component to form a cementitious nanocomposite. The oxygenfunctionalization of GnP surfaces that occurs during chemical surfaceenergy alteration introduces hydrophilic groups on the free surfaces ofgraphite sheets. This improves the chemical compatibility of the GnPwith their matrix by enhancing their wetting or adhesion characteristicsand reducing their tendency to agglomerate while in the cementitiousnanocomposite. In addition, CCFs are as reactive as functionalizedcarbon nanoparticles in hydrating cement, but their reaction does notresult in the mechanical reinforcement of the cementitious nanocomposite(i.e., does not result in improved interfacial adhesion) used to makethe cement. Rather, it is the chemical reaction of CCF-free carbon GnPsthat enhances the interfacial mechanical bonds for improved interfacialadhesion. Moreover, edge planes of GnP are highly reactive sites, whilethe basal planes are inert. The functional groups (e.g., carboxyl groupssuch as —COOH or —C═O) induced through oxygen functionalization of theGnP surface tend to be more easily formed on the GnP edge rather than onthe basal plane. The formation of functional groups on the GnP edgesameliorate the mechanical properties of the cementitious nanocompositeby enhancing interfacial adhesion. Enhancing interfacial adhesion isdefined herein as generating higher bond strength between the modifiedGnPs and the hydrations (C—S—H, calcium hydroxide, etc.) of cement.

In addition to using modified GnPs to influence the dispersionuniformity and interfacial adhesion of the GnPs in the cementitiousnanocomposite, the concentration of the modified GnP can be adjusted tofurther enhance the mechanical and durability characteristics.

In addition to the physical surface alteration and/or chemical surfaceenergy alteration, other dispersion techniques can be used to augmentthe dispersion methods disclosed herein. This can include mechanicaldispersion via ultrasonication and high shear stirring, for example.

It should be noted that embodiments of the cementitious nanocompositecan include modified GnPs having altered surface energies via chemicalsurface energy alteration, physical surface alteration, or both. Forinstance, some embodiments of the cementitious nanocomposite can includemodified GnPs that have been modified via both physical surfacealteration and chemical surface energy alteration. Some embodiments ofthe cementitious nanocomposite can include modified GnPs, where some ofthe modified GnPs have been modified via physical surface alteration andsome of the modified GnPs have been modified via chemical surface energyalteration.

EXAMPLE 1 Covalent Chemical Treatment: Acid Functionalization

A mixed solution of nitric (70 wt. %) and sulfuric (96 wt. %) acids witha volume ratio of 1:3 were provided. 1 wt. % GnPs were added to thesolution, and the mixture was stirred for 4 hours at 80° C. Then, GnPswere washed by deionized (DI) water and 5% HCl. To this end, afteradding DI water or HCl to the mixture, it was held still for 24 hr, andthen the top part of the diluted mixture was carefully removed. Thiswashing process was repeated if the sulfate ions were detectable in thediluted mixture. The process of washing by acetone was continued toremove CCFs. These fragments are organic molecules consisted ofcondensed aromatic graphitic rings with several functional groups areformed during GNP functionalization. CCFs are as reactive asfunctionalized carbon nanoparticles with hydrating cement but theirreaction does not result in the mechanical reinforcement of the producedcomposite. It is the chemical reaction of CCF-free carbon nanoparticleswith hydration products that is significant in forming interfacialmechanical bonds. Therefore, GnPs were iteratively washed bycentrifugation until clean and colorless diluted mixture is obtained.FIG. 4 shows the functionalized GnPs at different stages of acetonewashing.

EXAMPLE 2 Non-covalent Physical Treatment: Polymer Wrapping

Two different approaches for polymer wrapping of GnPs: (1) using thepoly acrylic acid (PAA) and (2) applying the combination of PVP and SDSwere evaluated.

PAA

1 gram of GnPs were added to 133 mL DI water. 10% PAA by weight of GnPswas also added to the mixture. The mixture was sonicated for 30 min inambient temperature and stirred overnight. Sonication was performedusing a Fisher Scientific, FS30D with the ultrasonic power of 130 W andthe operating frequency of 40 kHz. Then, the mixture was filteredthrough 0.2 μm filter and followed by deionized water washing for threetimes to remove excess PAA.

Combination of PVP and SDS

1 gram of GnPs were dispersed in 400 mL DI water with the aid of 1% SDSconcentration

$\left( {{\frac{M_{SDS}}{1000\mspace{14mu}{mL}\mspace{14mu}{water}} \times 100} = {1\%}} \right).$1% by weight of

${PVP}\mspace{14mu}\left( {{\frac{M_{PVP}}{M_{w} + M_{SDS} + M_{PVP} + M_{GnPs}} \times 100} = {1\%}} \right)$was also mixed to the blend. M_(x) (x=PVP, SDS, GNPs, and w) is the massof the corresponding material x, for example, M_(w) is the mass ofwater. The mixture was sonicated for 30 min. Then, the mixture wasincubated at 50° C. for 12 hours. GnPs were then filtered through a 0.2μm filter, washed with DI water and this was followed to remove anyresidual SDS. FIG. 5 shows the final obtained filter cake after theprocedure of polymer wrapping.

EXAMPLE 3 Preparation of Cement Slurry

The modified GnPs were re-dispersed in the amount of distilled waterrequired to prepare the cementitious matrix. The mixture of water andacid functionalized GNPs were sonicated for 1 hr and stirred for 3 hr.The mixture of water and modified GnPs by polymer wrapping weresonicated for 2 hr and followed by stirring overnight. Then, API class-Gcement was added to the mixture in the mixer. The ratio of water/cementwas 0.44 by weight. For the case when GnPs were modified using polymerwrapping, the nanoadditive concentration of 0.13% by weight of cement(BWOC) was examined. For the case when GnPs were subjected to the acidfunctionalization procedure, the effect of different concentrations ofGnPs was examined on the overall behavior of cement. To this end, thefollowing volume ratios were considered,

${{\frac{V_{GNPs}}{v_{{dehydrated}\mspace{14mu}{cement}}} \times 100} = {0.4}},{0.2},$and 0.1%, in which, V_(x) is the volume of the material x, GnPs or thedehydrated cement.

Studies were conducted on the cement slurry prepared above to examinethe effects of surface modified GnPs on the overall mechanicalproperties of the nanocomposite cements. The ultimate compressivestrength and the modulus of toughness of cement samples were calculated.Toughness is important specially to improve post-failure behavior of thecement, preventing unstable brittle fracture propagation. Inasmuch asthe compressive strength alone is not enough to indicate a cementsystem's ability for providing zonal isolation throughout the lifetimeof a well and after abandonment, additional experiments were conductedto better understand the mechanical response of cement systems todownhole conditions. To this end, rheological properties of cementsamples at two different temperatures of 120° F. and 190° F. as well asthe room temperature 73.4° F., thickening time under high-pressurehigh-temperature (HPHT) conditions, free fluid, shear bond strength,flexural strength, and depth of penetration into narrow spaces weremeasured.

Ultimate Compressive Strength

To measure the compressive strength, cubic samples of cement composed ofmodified GnPs were prepared. Cubic cement samples with 2×2×2 in.dimensions were prepared and cured under 3000 psi and 190° F. for 24 hrthen examined using MTS machine. Both surface modification methods, acidfunctionalization and polymer wrapping, were examined as to theireffects on the overall compressive strength of nanocomposite cementpastes. For each concentration of GnPs, two cubic samples are tested.

FIG. 6 is a Displacement v. Force plot examining how differentconcentrations of functionalized GnPs by acid modification affect theoverall compressive strength of cement. Different volume ratios ofGnP/dehydrated cement, 0.1, 0.2, and 0.4 Vol. % were considered As seenin FIG. 6 , these volume ratios of GnP almost result in the samemagnitude of compressive strength, about 42% greater than thecompressive strength of the plain cement sample. But different uniaxialdeformations happen at the ultimate compressive strength. In thefollowing, the displacements achieved at the peak load corresponding tothe modified cement samples are compared with that of the plain cement.The displacements corresponding to 0.1, 0.2 and 0.4 Vol. % of GnPsincrease, respectively, about 44%, 74% and 56% in comparison to that ofthe plain cement. For further comparison, the effect of differentconcentrations of GnPs on the modulus of toughness of the cement pasteis also examined. To this end, the stress-strain curves corresponding tothe results presented in FIG. 6 are first plotted and, then, the areaunder the curve is calculated and tabulated in Table 1. As it is seenfrom Table 1, the cement prepared using modified GnPs shows slightlygreater toughness than the plain cement. The maximum moduli of toughnessare corresponding to 0.1 and 0.2 Vol %.

TABLE 1 Comparison between the toughness of plain cement and cementscomposed of different concentrations of GNPs. Toughness (MJ · m⁻³) Plain0.1 Vol % 0.2 Vol % 0.4 Vol % 4.0512 4.1724 4.1362 4.0635

FIG. 7 presents the compressive behavior of prepared cement pastes usingphysically modified GnPs. This figure includes both methods of polymerwrapping: (1) using PAA and (2) applying the combination of PVP and SDS.For the samples fabricated from GnPs modified by PAA, the compressivestrength increases up to 23.67% in comparison with the plain sample. Butas it can be seen from FIG. 7 for the above-mentioned samples, the forcedrops down sharply after reaching to the ultimate load, exhibiting thebrittle behavior. Examination of the samples prepared using modifiedGnPs by the combination of PVP and SDS reveals that the compressivestrength does not improve, however the brittle behavior after reachingto the ultimate strength does not happen. In other words, surfacemodification of GnPs using the combination of PVP and SDS slightlyimproves the ductility of the cement paste in contrast to the ultimatestrength which decreases. The more ductile the cement is allows for moreloading cycles and a certain degree of deformation before reachingfailure.

Rheological Characterization

The rheological characterization of the modified cement slurries by GnPswas also examined. The deformation and flow behavior of cement slurriesat room temperature as well as two elevated temperatures were studies tounderstand cement rheology in deep-well cementing operations. Fornon-Newtonian fluids, the relationship between the shear stress, τ andthe shear rate, {dot over (γ)} in the steady laminar flow can bedescribed by mathematical models. In the well cementing industry, thefollowing models are commonly used,

$\begin{matrix}{\left. 1 \right)\mspace{14mu}{Power}\text{-}{law}\mspace{14mu}{model}} & \; \\{{\tau = {k\;{\overset{.}{\gamma}}^{n}}},} & (1) \\{{\mu = {k\;{\overset{.}{\gamma}\;}^{n - 1}}},} & (2) \\{\left. 2 \right)\mspace{14mu}{Bingham}\mspace{14mu}{plastic}\mspace{14mu}{model}} & \; \\{{\tau = {\tau_{y} + {\mu_{p}\overset{.}{\gamma}}}},} & (3) \\{{\mu = {\mu_{p} + \frac{\tau_{y}}{\overset{.}{\gamma}}}},} & (4) \\{\left. 3 \right)\mspace{14mu}{Herschel}\text{-}{Bulkley}\mspace{14mu}{model}} & \; \\{{\tau = {\tau_{y} + {k\;{\overset{.}{\gamma}}^{n}}}},} & (5) \\{{\mu = \frac{\tau_{y} + {k\;{\overset{.}{\gamma}}^{n}}}{\overset{.}{\gamma}}},} & (6)\end{matrix}$

where, μ is the viscosity of fluid, determined as the ratio of the shearstress, τ to the shear rate, {dot over (γ)}. The shear stress can beproportional to the friction pressure gradient or friction losses, andthe shear rate is the velocity gradient of fluid perpendicular to thefluid movement. The Bingham plastic model requires two parameters, theyield stress, τ_(y) and the slope of the line, known as the plasticviscosity μ_(p). This model presents the behavior of fluid which remainsunsheared until the applied stress reaches the minimum value, τ_(y)known as Bingham yield stress. The SI units of τ_(y) and μ_(p) are,respectively, [τ_(y)]=Pas and [μ_(p)]=Pa. In the case of power-lawmodel, k is the flow consistency index with the SI unit [k]=Pa s^(n),and n, the dimensionless parameter, is known as the flow behavior index.Based on this model, flow commences when the shear stress exceeds to theyield stress and, then, follows with the power-law behavior.

The Fann viscometer model 35 was used to measure the gel strength andthe rheological properties of cement slurries. The cement slurry wassheared between an outer rotor and an inner cylinder. The rotor spinedat different rotational speeds, Ω=300, 200, 100, 6, 3 rpm and exerted atorque to the inner cylinder by slurry. This torsional deflection wasindicated on a dial which is read. For each rotational speed, Ω, themeasured values of ramp-up and ramp-down dial readings, θ, wereaveraged. Then, data analysis was performed by converting Ω and θ toshear rates and shear stresses at the inner cylinder, respectively. Tothis end, the following equations were used{dot over (γ)}=16.82Ω, when [Ω]=rad/s,  (7)or{dot over (γ)}=1.705Ω, when [Ω]=rpm.  (8)τ=0.5109θ, when [θ]=Pa,  (9)orτ=1.067θ, when [θ]=lbf/100 ft ²  (10)

The test was conducted at the room temperature T=73.4° F. and two otherelevated temperatures of T=120° F. and 190° F. The cement slurry wasprepared by adding 0.2 Vol. % functionalized GnPs. The obtained data arepresented in Table 2.

TABLE 2 Viscometer readings for the cement slurry containing 0.2 Vol. %functionalized GnPs at three different temperatures, T = 73.4, 120, and190° F. θ at 73.4° F. θ at 120° F. θ at 190° F. [lbf/(100 ft²)][lbf/(100 ft²)] [lbf/(100 ft²)] Ω Ramp- Ramp- Ramp- Ramp- Ramp- Ramp-(rmp) up down Ave. up down Ave. up down Ave. 300 131 138 134.5 276 260268 295 294 294.5 200 119 125 122 254 237 245.5 284 279 281.5 100 104110 107 215 205 210 235 236 235.5 6 39 38 38.5 60 35 47.5 37 25 31 3 2117 19 35 21 28 20 17 18.5

For the purpose of data analysis, the converted measurements of shearstresses corresponding to each shear rate were fitted to themathematical models of power-law, Bingham plastic, and Herschel-Bulkleymodels. FIGS. 8A-8C show how these models fit the shear stress-shearrate curves.

For further clarification, the rheological model parameters are alsogiven in Table 3. It is found that, fitting of the Herschel-Bulkleymodel at 73.4° F. and 120° F. results in τ_(y)=0, which presents thepower-law model. However, for the case when T=190° F., theHerschel-Bulkley model leads to τ_(y)=1.3919 Pa. From Table 3, it can beconcluded that the Herschel-Bulkley model fits the rheological behaviorof the cement slurry better than the Bingham plastic and power-lawmodels. Considering the Herschel-Bulkley model, it is seen from FIGS.8A-8C that the viscosity decreases as the shear rate increases,indicating shear thinning fluid. In the case of power-low fitting, theindex n is also obtained less than 1, which predicts the behavior ofshear thinning fluid. Moreover, from Table 3, it is found that the indexn corresponding to the power-law model increases from 0.3997 to 0.6303by raising the temperature from 73.4° F. to 190° F. Note that, when theindex n is 1, the fluid is Newtonian.

TABLE 3 Rheological Model Parameters for the prepared cement slurry atthree different temperatures, T = 73.4, 120, and 190° F. CorrelationFitting model Coefficient, R n τ_(y) (Pa) k (Pa · s^(n)) μ_(p) (Pa · s)T = 73.4° F. Bingham plastic 0.9163 — 13.287 — 0.0299 Power-law 0.98630.3997 — 2.9883 — Herschel-Bulkley 0.9889 0.3411 0.0  4.0835 — T = 120°F. Bingham plastic 0.9190 — 14.867 — 0.1166 Power-law 0.9800 0.4953 —3.3949 — Herschel-Bulkley 0.9881 0.3891 0.0  6.1081 — T = 190° F.Bingham plastic 0.9077 — 13.368 — 0.1385 Power-law 0.9596 0.6303 —1.7359 — Herschel-Bulkley 0.9771 0.4632 1.3919 4.3168 —

The shear stress-shear rate curves corresponding to differenttemperatures are compared in FIG. 9A. As it is found from this figure,when the cement slurry is composed of GNPs the shear stress increases asthe temperature increases, while the increase of temperature decreasesthe shear stress for the plain cement. The viscosity of the cementslurry is also calculated from Eq. (6) using the shear stress-shear ratecurve fitted by the Herschel-Bulkley model and plotted in FIG. 9B.Therefore, the change of the viscosity of cement slurries versustemperature in the presence or absence of GNPs is understood. FIG. 9Bpresents the role of temperature and the presence of GNPs on the changeof viscosity. For the cement slurry including acid-functionalized GNPs,it is in general obtained from FIG. 9B that the elevation of temperaturefrom the room temperature increases the viscosity of the slurry.Although, the increase of temperature decreases the viscosity of theplain cement.

The gel strength, which is a measure of the attractive forces betweenthe particles in a fluid under static conditions, was also measured. Tothis end, the sample was thoroughly stirred at 300 rpm. Then, therotational speed was set to be equivalent to 3 rpm, and the viscometeris turned off for 10 minutes. After turning on the viscometer, the peakdial reading presents the gel strength. For the sample prepared by 0.2Vol % modified GnP, the gel strength was measured as 17 lbf/(100 ft²)which is equivalent to 8.14 Pa. For the sake of comparison, the gelstrength corresponding to the plain cement is also measured which isobtained as 22 lbf/(100 ft²) equivalent to 10.53 Pa. Therefore, for thecement sample including modified GnP, the gel breaks at slightly lowershear stress in comparison to the neat cement. Inasmuch as, static gelstrength is related to the annular fluid migration, the lowermeasurement corresponding to the modified cement shows its bettermovability than the plain cement.

Thickening Time Characterization

Standard HPHT thickening time tests were conducted to measure the lengthof time the prepared cement slurry remains in the pumpable state at thetemperature 52° C. and the pressure 5160 psi. The cement slurryconsistency was measured in Bearden units of consistency (B_(c)), whichis a dimensionless quantity which cannot be transformed directly toviscosity values. The thickening time was measured as the time at whichthe cement slurry reaches a consistency of 100 B_(c). The time when theconsistency starts to increase is called the point of departure. Cementsamples having different concentrations of GnP, 0.10, 0.20, and 0.40Vol. % were examined. The results corresponding to 0.10, 0.2 and 0.40Vol. % concentrations of GnP are given in FIGS. 10A-10C, respectively.Furthermore, the consistency of different cement samples havingdifferent concentrations of GNP is also compared in FIG. 11 . As it isseen from FIGS. 10-11 , the presence of GNPs slightly increases thethickening time. The thickening time measured for the neat cement slurry(including no GNPs) is about 01 h:35 m, while it becomes about 01 h:40m, 01 h:55 m, and 01 h:45 m, for 0.1, 0.2, and 0.4 vol. % concentrationsof GNPs, respectively.

How the concentration of GnP affects the consistency-time behavior ofthe prepared cements is discussed in the following paragraph.

It is seen from FIG. 11 , the departure time for the neat cement slurryand the cement slurry composed of 0.1 vol. % of GNPs is approximately 01h:05 m. The increase of GNPs concentration from 0.1 vol. % to 0.2 vol. %decreases the departure time to about 55 m. Immediately after departuretime, the viscosity of slurries including 0.2 and 0.4 vol. %concentration of GNPs shows a small jump. Showing a sudden increase inthe slurry viscosity brings the attention toward the potentiality offabrication of right-angle set (RAS) cement slurries by tuning theconcentration and surface properties of GNPs. Such systems are sometimescharacterized by standard HPHT thickening time tests, in such a waythat, the slurry viscosity increases from a low consistency to more than100 B_(c) within a few minutes. RAS cement slurries develop alow-permeability matrix which enables to significantly prevent gasintrusion up to the commencement of cement setting. It should be notedthat the mechanism of set involving for RAS cement slurries is not likethat of the high-gel strength systems. In general, it is the gelstrength properties that determines the ability of a setting cementslurry to resist the influx of well bore fluids.

Free Fluid Measurement

When the cement particles are not dispersed completely, they interactwith each other through the electrostatic forces, leading to theformation of a flocculated structure. Typically, the weight of thecement particles is transmitted to the bottom of slurry by the gellattice and, consequently, a structural deformation happens within thecement. Due to this phenomenon, water is squeezed out of the lower partsof the slurry and migrates upward the less-stressed upper layers. Thecapacity of the higher layers to accommodate the additional water islimited and, thus, a layer of water may accumulate at the top of thecement slurry. This separation and movement of water from the bottom ofthe slurry upward the higher layers can remain channels within thecement, which will promote the probability of gas migration and impairzonal isolation. In other words, the free water could coalesce to form acontinuous channel on the upper side of the hole and develop aprivileged path by which the gas may migrate.

Based on this phenomenon, a study was conducted to measure thisseparation tendency for the slurries made of different concentrations ofGnPs, 0.1, 0.2 and 0.4 Vol. %, using the procedure documented in API RP10B. To this end, 250 mL of the prepared slurry was poured into agraduated cylinder with 0° of inclination and held still for 2 hr. Toprevent water evaporation, the top of the cylinder was covered. Then,the amount of water accumulated at the top of the slurry was measured.For the sake of comparison, the study included an experiment for theplain cement slurry without GnPs. For this case, the free fluid wasmeasured as 1.0 mL. However, when the cement slurries were prepared byadding the surface modified GnPs, these nanoadditives do not induce anyfree water separation. In other words, zero free fluid was measured fordifferent concentrations of GnPs, 0.1, 0.2 and 0.4 Vol. %. It isnoteworthy to mention that if GnPs are not efficiently dispersed withinthe cementitious matrix, a nonzero free fluid is obtained. For example,an experiment with 0.2 Vol. % of GnPs was conducted where they were notcompletely dispersed. The resultant measurement of free fluid was about0.6 mL for this case, while it becomes zero when GnPs were efficientlydispersed.

Shear Bond Strength Improvement

One of the primary goals of the cement sheath in a wellbore is providingzonal isolation. As noted above, the cement of the cement sheath issubjected to severe conditions during the life of a producing oil/gaswell that can affect the permeability of the cement matrix. Cracking,debonding, and shear failure are the main contributors to theseconditions. Lack of strong bonding between cement and formation can leadto the shear failure, resulting in the complete failure of the cementsheath. The subsidence and movement of formation as the oil or gasreservoir is depleted, as well as vibrations, caused by downhole pumpsor gas-lift operations increase the effective stress around a wellboreand may cause shear failure.

A study was conducted to examine the effect of acid-functionalized GnPon the shear bond strength at the cement-shale formation interface. Tothis end, a modified push-out test was employed. Setup includes acylindrical shale core, cement, and steel pipe, as shown in FIG. 12A.Dimensions of steel casing is presented in FIG. 12B, and the diameter ofthe formation core was 1 in. To mimic downhole conditions, cement wascured under high pressure and high temperature conditions for 24 hours.Then, the prepared sample was subjected to a push-out force by thedisplacement control loading mode with 0.3 mm/min rate. Part 4 in FIG.12A illustrates how the load is applied to the shale core using a steelcylinder.

The results obtained from push-out tests corresponding to the cementslurries with different GNP concentrations of 0.1, 0.2, and 0.4 vol. %as well as the plain cement are compared in FIG. 13 . As it is seen,cements composed of acid-functionalized GNPs show higher shear bondstrength in comparison with the plain sample. The highest improvement isachieved by adding 0.1 and 0.2 vol. % of GNPs to the cement slurry,which results in the increase of the shear strength about 71% and 175%as compared to the plain cement, respectively.

Flexural Strength Improvement

The cement sheath under harsh downhole conditions will be exposed to acombination of tension, compression and shear loads, which willparticularly threat the integrity of wellbore due to the inherent lowtensile strength of cement. Flexural strength of materials is one of theprincipal properties to evaluate their mechanical response under tensileloading.

Three-point flexural tests are conducted in the lab to examine howacid-functionalized GNPs affect the tensile strength of the preparednano-reinforced cements. Beam-shape cement samples with the width of b=1in., the depth of d=0.5 in., and the length of 5 in. satisfyingTimoshenko beam criteria are prepared. The span length, the distancebetween supports in the three-point load test, is L=4 in. Adisplacement-controlled loading with the rate of 0.1 mm/min is utilized.

FIG. 14 illustrates the experimental response of cement beams composedof different concentration of GNPs, 0.1, 0.2, 0.4 vol. %, as well as theplain cement under flexural test. As it is seen, 0.2 vol. % ofacid-functionalized GNPs significantly increases the flexural strengthof the cement nanocomposite as compared to the plain cement. Precisely,0.2 vol. % concentration of acid-functionalized GNPs increases theflexural strength of the nano-reinforced cement about 209% in comparisonto the plain cement sample.

SEM Imaging to Evaluate Microstructural Improvement

FIGS. 15A-15D illustrate SEM images captured from different locations onthe fracture surface of cement nanocomposite composed of 0.2 vol. %surface-modified GNPs. The effect of acid-functionalized GNPs on thepore refinement is examined. FIG. 15A displays a pore on the fracturesurface of the cement nanocomposite, revealing the formation of thinplatelets inside the pore. For a further elaboration, the magnifiedmicrostructure of these platelets is also provided in FIGS. 15B and 15C.The polyhedral form of hydration crystals can be seen in FIG. 15B, andthe formation of a thin platelet with the thickness of about 2 μm can beobserved in FIG. 15C. The production of entangled cloud-like networksinside the pores/voids was also captured, as shown as an example in FIG.15D.

For the plain cement, none of the above-mentioned interior features isobserved.

Injection Test for Squeeze Treatment

The appropriateness of the cement slurry composed of surface-modifiedGNPs for the squeeze treatments is also examined.

Squeeze cementing is the process of placing cement slurry from surfaceto downhole to penetrate through narrow spaces behind the casing and/orperforations placed in the casing. Mainly, a squeeze job is conducted torepair a faulty primary cement job, to isolate formation intervals, toalter formation characteristics, and to finally repair some casingproblems. Of some reasons requiring squeeze cement treatments are thepresence of micro-annuli at the casing-cement or the cement-formationinterfaces, improper displacement of drilling-fluid, and gas influx intothe cemented annulus.

Better penetration into narrow spaces behind the casing is one of theprerequisite of squeeze-cement.

FIG. 16 illustrates an exemplary set-up for injection cell. It iscomposed of a channel with the thickness of 120 μm and the width of 1in. The channel is made over a filter paper. Then, to resemble theformation a filter paper is laid over a porous layer of aluminum whichis mimicking the formation. A transparent and very smooth plate (made ofglass) is placed on the channel to represent the casing. A hole isdrilled in the glass plate to inject the cement slurry into the channelusing a syringe pump.

FIGS. 17A-17B gives how deep cement slurry can penetrate into thechannel for the plain cement and the cement slurry including 0.2 vol. %surface-modified GNPs, respectively. As it is seen, about 250% morepenetration is achieved for the cement nanocomposite as compared to theplain cement.

It should be understood that the disclosure of a range of values is adisclosure of every numerical value within that range, including the endpoints. It should also be appreciated that some components, features,and/or configurations may be described in connection with only oneparticular embodiment, but these same components, features, and/orconfigurations can be applied or used with many other embodiments andshould be considered applicable to the other embodiments, unless statedotherwise or unless such a component, feature, and/or configuration istechnically impossible to use with the other embodiment. Thus, thecomponents, features, and/or configurations of the various embodimentscan be combined together in any manner and such combinations areexpressly contemplated and disclosed by this statement.

It will be apparent to those skilled in the art that numerousmodifications and variations of the described examples and embodimentsare possible in light of the above teachings of the disclosure. Thedisclosed examples and embodiments are presented for purposes ofillustration only. Other alternate embodiments may include some or allof the features disclosed herein. Therefore, it is the intent to coverall such modifications and alternate embodiments as may come within thetrue scope of this invention, which is to be given the full breadththereof.

It should be understood that modifications to the embodiments disclosedherein can be made to meet a particular set of design criteria. Forinstance, any of the surface energy alteration techniques, dispersionmethods, or any other component or operating parameter can be anysuitable number or type of each to meet a particular objective.Therefore, while certain exemplary embodiments of the apparatus andmethods of using and making the same disclosed herein have beendiscussed and illustrated, it is to be distinctly understood that theinvention is not limited thereto but may be otherwise variously embodiedand practiced within the scope of the following claims.

What is claimed is:
 1. A method for improving properties of cement, themethod comprising: generating a cementitious nanocomposite underhigh-pressure and high-temperature conditions, the cementitiousnanocomposite comprising modified graphite nanoplatelets (GnP), themodified GnP being functionalized by introduction of hydrophilic groupson its surface via chemical surface treatment by generating a solutioncontaining nitric acid and sulfuric acid, adding GnP to the solution,stirring the solution, washing GnP with deionized water and HCl, andwashing GnP by acetone to remove carboxylated carbonaceous fragments(CCFs); using the cementitious nanocomposite as a component of cement,wherein the modified GnP have a reduced tendency to agglomerate whilebeing part of the cement due to the hydrophilic groups; and wherein theproperties of the cement include integrity, ductility, toughness,compressive strength, tensile strength, flexural strength, shear bondstrength, microstructure, permeability, viscosity, Rheology, thickeningtime, and free fluid formation.
 2. The method recited in claim 1,wherein the hydrophilic groups allow for more uniform dispersion ofmodified GnP throughout the cement.
 3. The method recited in claim 2,wherein formation of the hydrophilic groups on GnP edge surfacesincreases interfacial adhesion between modified GnP and the cementmatrix.
 4. The method recited in claim 3, wherein the increasedinterfacial adhesion provides higher bond strength between modified GnPsand hydrations of the cement.
 5. The method recited in claim 4, wherein,when the cement is used as a cement sheath in a wellbore, the increasedinterfacial adhesion prevents or inhibits debonding of the cement from acasing of the wellbore and/or debonding of the cement from surroundingrock formations of the wellbore.
 6. The method recited in claim 2,wherein, when the cement is used as a cement sheath in a wellbore, themore uniform dispersion of modified GnP prevents or inhibits diffusionof trapped oil and/or natural gas through the pore network and/ormicro-cracks of the cement.
 7. The method recited in claim 1, whereinmodified GnP concentration is within a range from 0.10 Vol. % to 0.40Vol. % with respect to the volume of the dry cement.
 8. The methodrecited in claim 1, wherein any one or combination of the following: thecement is used at high-pressure and high-temperature for a primaryoilwell, a gas-well, a geothermal well, and/or a waste disposal well;the cement is used for primary cementing of a primary oilwell, agas-well, a geothermal well, and/or a waste disposal well; the cement isused for remedial treatment of a primary oilwell, a gas-well, ageothermal well, and/or a waste disposal well; the cement is used forplugging and abandonment of a primary oilwell, a gas-well, a geothermalwell, and/or a waste disposal well; and the cement is in presence ofwater-based mud and/or oil-based mud.
 9. The method recited in claim 1,wherein use of the cement improves cement bonding to formation andcasing with the use of spacer fluids or without the use of spacerfluids.
 10. The method recited in claim 1, wherein use of the cementreduces leakage and mechanical failure by reducing the number and sizeof microcracks and voids in a cement matrix.
 11. The method recited inclaim 1, wherein use of the cement provides better penetration intoflaws and narrow open spaces in the cement.